You can get coded tags for stuff at travel stores. You register at the website and if someone finds your stuff (not just keys) they call the number on the tag and they tell them what to do with it. Got one on my keyring.

i love that its a hard drive magnet.. but i have a store-bought hide-a-key on my car... in a difficult place to find. difficult enough that i thought I lost it on the road somewhere until I looked again a couple months later!

i was thinking the same thing, but that could make it difficult to remove (since if you lost your keys you may have lost your knife), and gunk will accumulate inside of the heat shrink.
i think a pop rivet through the fob and magnet will hold things pretty well, whiel still allowign use of the key and the brass of the key is reasonably non reactive, and unless it's getting battered by gravel in your wheel well, it should be ok on it's own. maybe a wrap or two of saran wrap over the entire thing.

Simple solution:
Jewelry or bead baggie (The little ones that look like they're for dope). Put the whole thing into the baggy, then wrap the baggy in just one or two loops of tape to make it tight.
No goop on key, and still reasonable to open.

True. I'm not saying put a florescent green baggie under your car. What I mean is that most thieves now to look for spare keys under the car. If they see a baggie wrapped in tape, that is an obvious sign.

Read the instructable again. And look at the pics. The key is connected to the magnet with a steel split ring. The tape is only there "to prevent the key from flapping around." Actually, you could just wrap the thing with a few tight turns of saran wrap and call it good.